Clive wrote:
The waste water pipe from the bathroom sink passes through the wall
into a 90 degree bend on the external wall. Now as far as I can tell
the bend is a push fit rather than a screw threaded type used on other
waste water pipes at my house. Now this makes the fitting very "slim-
line". However, the bend is partially "embedded" within a mortar
rendering and the external "down" pipe has worked loose. I have pushed
the external pipe back into the bend (tricky because it is jammed
behind a vertical soil pipe). However, I don't think the pipe has
fully "engaged" in the bend and I'm pretty sure it will work loose
again. What's the best way to repair this permanently? I'm thinking of
chipping away the morter and replacing the bend with some other
fitting - but I'm not sure what to use

In my opinion the final result should be a solvent-weld joint. Made
properly (and it's not especially hard) this will be permanent, robust,
and neat. However, you can only weld to the right kind of pipe, and if
your existing pipe really is push-fit then it will not be suitable.

That said, push-fit sounds a slightly unusual choice in your situation,
and I wonder whether it's actually a solvent-weld fitting that wasn't
properly welded and has failed. I had something similar on the front of
my house; you could push the pipe into the socket where it would stay
for a while before falling out again. When I finally got round to fixing
it I cleaned up the inside of the socket and the outside of the pipe
with fine sandpaper, and welded it properly. It's been completely solid
since.

Pete